Waterborne vinyl acetate/ethylene (VAE) polymers prepared by emulsion polymerization are extremely important commercial materials for use in paint and adhesive formulations. Because of the dual phase nature of these materials, they require a stabilizing package to maintain water dispersibility. Conventionally, stability is maintained through the addition of surfactants or water soluble polymers such as polyvinyl alcohol or cellulose.
A goal in producing one type of vinyl acetate/ethylene copolymers for paint applications, including semi-gloss, is one of simultaneously providing low minimum film forming temperature (MFFT) in the emulsion and block resistance in a paint incorporating the emulsion. One generally seeks a minimum film forming temperature (MFFT) of less than 5.degree. C. in the emulsion and high block resistance values in the paint. Block resistance is the measure of the adhesiveness of the paint to itself after drying. When a paint which has low block resistance is applied to two substrates and the two substrates come into contact, the two substrates will adhere to one another.
In order to reduce MFFT and retain high block resistance, solvents traditionally have been added to the paint formulations. Many solvents are considered volatile organic compounds (VOC) and therefore undesirable from an environmental perspective. Solvents act to lower the MFFT of the emulsion polymer by temporarily reducing the glass transition temperature. On evaporation of the solvent, the paint becomes block resistant by virtue of the hardness of the polymer. Without solvent added, the high Tg of the polymer prevents film coalescence.
Emulsion polymers have been prepared in the presence of water dispersible polyurethanes. Typically, these polymers are referred to as hybrids combining features of each while differing from the features of blends. For the most part, such water dispersible polyurethanes have been anionic polyurethanes incorporating amine neutralized carboxyl functionality. More recently, some nonionic polyurethanes have been utilized to stabilize the emulsion polymerization of a variety of monomers.
Representative patent literature which show water dispersible polyurethane/hybrid systems and mixtures of water dispersible polyurethanes and vinyl and acrylic emulsions are as follows:
U.S. Pat. No. 4,927,876 discloses two types of aqueous polymer dispersions for use as coating compositions. The dispersions comprising a water dispersible polyurethane containing diphenylmethane diisocyanate and a vinyl polymer. The water dispersible polyurethane component is prepared by forming an isocyanate terminated prepolymer having carboxyl functionality incorporated therein and chain extending with an amine. In forming the aqueous dispersion, the vinyl polymer may be blended with the water dispersible polyurethane or a vinyl monomer may be polymerized in the presence of the aqueous polyurethane dispersion. A variety of monomers suited for polymerization are suggested and these include acrylates and hydroxyacrylates.
U.S. Pat. No. 4,190,566 discloses nonionic, water dispersible polyurethanes having a linear molecular structure. The nonionic, water dispersible polyurethanes are formed by reacting organic diisocyanates with difunctional organic compounds having molecular weights from about 300 to 6,000 in the presence of components having hydrophilic groups, e.g., conventional polyether and polyester polyols, guaranteeing the dispersibility of the polyurethanes. The hydrophilic components are based on reacting alkylene oxides with monofunctional alcohols or any alternative reacting monoisocyanate with the polyether alcohol. Optionally, the water dispersible polyurethanes may be blended with other vinyl polymers for the formulation of coatings.
WO 94/13726 discloses a class of hydrophilic polyurethanes which are water dispersible and useful as protective colloids in combination with conventional nonionic surfactants for use in stabilizing the emulsion polymerization of monomers such as vinyl acetate and vinyl chloride. These polyurethanes are high viscosity, water dispersible polyurethanes, and these are formed by reacting aliphatic or aromatic diisocyanates with polyethylene glycols. The latter are polyaddition products of ethylene oxide and a variety of divalent alcohols, a representative is the reaction product of a polyethylene glycol and meta-tetramethylxylene diisocyanate. The patent also discloses the use of these high viscosity polyurethanes in producing polyvinyl acetate water-resistant films.
WO 96/28489 and its equivalent, DE 195 08 856, disclose the use of water soluble-nonionic polyurethanes as protective colloids of the type described in WO 94/13726 for the emulsion polymerization of a variety of monomers. Representative monomers suited for forming aqueous dispersions include acrylic and methacrylic acid esters, acrylic and methacrylic acids as well as their hydroxy alkyl esters, vinyl acetate and copolymers of vinyl acetate and ethylene. The patentees also point out that redispersible powders can be obtained from these emulsions. Representative examples then show the use of the water soluble polyurethanes as protective colloids alone and in combination with surfactants for the polymerization of ethylene, vinyl acetate and N-methylolacrylamide.
U.S. Pat. No. 5,314,942 discloses the preparation of aqueous polymer dispersion containing vinyl polymer and a nonionic water-dispersible polyurethane having pendent polyoxyethylene chains as a stabilizer in emulsion polymerization. The nonionic polyurethanes having pendent polyoxyethylene chains are alleged to possess significant advantages over the anionic polyurethanes described in the art. Most of the examples show the polymerization of acrylic monomers in the presence of the nonionic polyurethane dispersion and sodium lauryl sulfate.
Representative literature and patents regarding minimum film forming temperature emulsions and block resistant paints are as follows:
S. T. Eckersley and B. J. Helmer, "Mechanistic Considerations of Particle Size Effects on Film Properties of Hard/Soft Latex Blends," J. Coatings Technology, 69(864), 97-107(1997). This article describes the use of appropriate weight ratios of hard and soft latices to yield coatings with improved block resistance and low MFFT. Low levels of small particle size hard latexes when blended with larger particle size soft latexes yielded optimum properties due to packing considerations.
M. A. Winnik and J. Feng, "Latex Blends: an Approach to Zero VOC Coatings," J. Coatings Technology, 68(852), 39-50(1996). This article describes the use of appropriate weight ratios of hard and soft latices to yield coatings with improved block resistance and low MFFT. Low levels of small particle size hard latexes when blended with larger particle size soft latexes yielded optimum properties due to packing considerations and longer drying times.
U.S. Pat. No. 4,283,320 discloses addition of a methacrylic acid--styrene (hard) copolymer to a VOC containing semigloss paint formulation incorporating a soft latex for improving block resistance.
U.S. Pat. No. 5,530,056 describes use of a poly(ethylene glycol) mono(meth)acrylate macromonomer with a molecular weight from about 175-1100 g/mole in an acrylic monomer containing emulsion to impart freeze-thaw stability and block resistance to the final coating.
U.S. Pat. No. 5,426,129 discloses reactive coalescents incorporated into emulsion polymers post-polymerization which allow for film formation followed by crosslinking.